The proposed research will evaluate the immunologic and environmental determinants of the risk of insulin dependent diabetes mellitus (IDDM). Specifically, we will: 1. Monitor the incidence of IDDM in Allegheny County, PA, over 25 years (1965-1990) as well as the world. 2. Assess the associations between specific serologic and molecular markers to the risk of IDDM in 90 multiple sibling families, 58 affected parent offspring families, and 60 single case families. 3. Evaluate the association between viral DNA insertion into the human genome and IDDM. 4. Continue to collect samples on first degree relatives of cases prior to conversion to IDDM.